First-Principles Calculations and Electron Density Topological Analysis of Covellite (CuS)

Abstract

Covellite (CuS) is an important mineral sulfide that can be used in many technological applications. It has a simple formula but a complex structure consisting of alternating layers of planar CuS3 triangles and CuS4 tetrahedrons with S-S bonds. Accurate first-principles calculations are performed for covellite structure (CuS), aiming to provide insights about its structural, mechanical and electronic properties and to unveil the nature of its chemical bonding. DFT and DFT+U methods have been used and showed to be sensitive to the correlation treatment (U value). Although it is not possible to extract a universal value of the U, this study indicates that U = 5 eV is an adequate value. The electronic structure analysis shows a significant metallic character due to p(S)-d(Cu) orbital interactions up to Fermi level. The projected density of states indicates that most of the contribution comes from the atomic orbitals in the [001] plane of the covellite, explaining the conductivity anisotropy observed experimentally. Topological analysis of the electron density was performed by means of quantum theory of atoms in molecules (QTAIM). Two different topological charges in Cu and S were calculated, confirming an ionic model with mix-charges. This mineral presents ionic degree of ∼ 32%. On the basis of the QTAIM analysis, the covalent character of S-S bond is confirmed, and the favored cleavage of CuS at the [001] surface might be at the Cu-S bond. The S atoms occupy most of the cell volume, and their contributions dominate the crystal compressibility: κ(S) ≈ κ(CuS).

Deposition of Highly Crystalline Covellite Copper Sulphide Thin Films by SILAR

“…The optical response of the CuS can be analyzed focusing in the different regions of the electromagnetic spectrum included in the measurements. Beginning with the infrared (IR) zone, the drop down of the T evidences the typical metallic behavior of CuS, in agreement with the literature . The presence of a high concentration of charge carriers (holes), originated in its crystalline structure, and the nanocrystalline nature of the films (great amount of grain boundaries), are the main sources of the great absorption of light in this zone.…”

“…It is worth mentioning that the bandgap reported for covellite in several works is in the vicinity of the values corresponding to such electronic transition in CdS (≈2.45 eV), making difficult to differentiate them. The metallic character of the CuS films corresponding to a highly degenerated semiconductor, is a reported behavior of the covellite CuS structure attributed to the large concentration of holes …”

“…Following the optical analysis of the T spectra, the SE measurements were analyzed to determine the behavior of the optical constants of the system. The crystalline structure of the CuS covellite phase is very complex, and the electronic structure calculations reported in the literature reflects such complexity in a band structure with several possible electronic transitions at energies above and below the bandgap . Thus, the presence of the critical points observed in the ellipsometric spectra (not shown here) suggests the influence of those electronic processes occurring as a result of the interaction of the light probe with the CuS films.…”

Near-infrared radiation absorption properties of covellite (CuS) using first-principles calculations

“…Figures 2 and 3 indicate that the energy band curves and the DOS curves pass through the Fermi energy (E F ) level at low temperature (T = 0 K), which is in good agreement with the results based on the standard all-electron linear augmented plane wave method. 13,14,21 Figure 3 also shows that both the top of the valence band (VB) and the bottom of the conduction band (CB) are mainly composed of Cu 3d and S 3p states. It should be noted that the scissor operators on both the electronic structure and the optical properties were not considered in our calculations.…”

“…6 Recently, CuS has attracted attention due to its localized surface plasmonic absorption in the near-infrared (NIR) region, making it a potential candidate for other applications including photothermal cancer therapy, [7][8][9][10] biomedical imaging 10,11 and solar radiation absorption. 12 Although several theoretical 13,14 and experimental 15,16 studies have been carried out in an attempt to understand the electronic structure, chemical bonding and NIR absorption of CuS, 16 the explanation behind its NIR absorption remains unclear.…”